When researching her first foray into children's fiction – the marvellous Montmorency novels – Eleanor Updale found herself deep beneath the streets of London in a public sewer. It's unlikely that any such strenuous preparations were required for Johnny Swanson, save perhaps a trip to the former sanatorium of Craig-y-Nos Castle in Wales, which is today, rather conveniently, a hotel. This latest novel is, I would suggest, pitched at a slightly younger audience than Montmorency (though a thoroughly enjoyable read for this adult), and is in an undeniably lighter vein.

The story has three main strands: the schoolboy Johnny Swanson earning income from the personal ads; the spread of tuberculosis in the 1920s; and the up-to-no-good skulduggery – what other kind is there? – that somehow bridges these first two strands.

Johnny gets into the personal ads business when he sends off a two-shillings-and-sixpenny postal order to find out the SECRET OF INSTANT HEIGHT. (Their capitals.) If you've no idea what two shillings and sixpence is, or a postal order come to that, you'll soon find out. Johnny doesn't have that much money, so he "borrows" it in the not-actually-asking-but-having-every-intention-of-paying-it-back sense. When buying the postal order from the local post office, he creates a fictional Aunt Ada, pretending that she's sick and will be spending the two-and-six to buy a train ticket to come and stay with him and his mother. The response (the secret of instant height) when it finally arrives, is not quite what Johnny expected. It is to stand on a box. He feels outraged and more than a little stupid.

Rather than seek revenge, he sees the potential in such a scheme, or rather scam. He soon starts coming up with advertisements of his own. To do this, his fictitious aunt has a new use: as the "adult" placing the ads for whom he is apparently running errands. When Johnny's mother is taken away and charged with a serious crime, "Aunt Ada" takes on an even more significant role. She's the nonexistent responsible adult supposedly looking after him, thus freeing him to do the best he can to prove his mother innocent.

At one time, a quarter of all UK deaths were attributed to tuberculosis and, right up until a cure was found in the 1940s, tens of thousands of people were affected every year, many of whom died. In 1929 – when this book is set – a supposed cure, known as Umckaloabo, was advertised in the press; an advertisement even less honest than Johnny's "official portrait of the king for one shilling". (Anyone who sent off for that one received a postage stamp. Well, there really was a picture of the king's head on it.)

When Johnny plays detective, don't expect a story of intricate, multilayered plotting and subtle subterfuge. Johnny Swanson is very much of the plot-exposition-as-dialogue-overheard-by-an-eavesdropper-conveniently-under-the-table-at-the-time school of detective fiction. There's no real need for the strenuous application of little grey cells here. Events move at a fair lick, and revelation follows revelation. Updale writes with such obvious relish that fun exudes from the ink on every page. This is real entertainment. Johnny Swanson is just the kind of book for which the term "joyous romp" was invented.